The invention relates to a sawing machine for sawing metal workpieces. Such a sawing machine comprises a machine frame and an upper saw part attached to the machine frame, said upper saw part carrying a sawing tool which is driven so as to generate a material-removing sawing movement. The upper saw part is movable relative to the sawing bench in order to execute a sawing feeding movement, extending in a cutting plane, of the sawing tool. The machine frame comprises a sawing tool and clamping jaws, which are movable back and forth, for positioning workpieces on the sawing bench. The sawing machine is finally also provided with a machine controller for controlling the movement sequences.
Sawing machines of the present type are generally realized as band sawing machines or circular sawing machines. The sawing tool can accordingly be a sawing band or a saw blade.
Metal workpieces which are sawn by way of sawing machines of the present type are generally so heavy that they cannot be placed manually on the sawing bench. Rather, they are supplied to the sawing bench by conveying means. For the actual sawing operation, the workpieces are clamped in the clamping jaws at the sawing bench and then the upper saw part is lowered in order to carry out the desired sawing cut.
The clamping jaws of a sawing machine of the present type are movable back and forth in order to position the workpieces for the sawing operation. At the beginning of a sawing operation, the clamping jaws carry out an advancing movement by a length which corresponds to the length of the workpiece part to be sawn off. Accordingly, at the beginning of a sawing operation, the workpiece has to be positioned in a zero position in which a front end side of the workpiece is located in the cutting plane. This is because it is only then that the advancing movement of the clamping jaws results in the cutting plane extending at the desired distance from the end side of the workpiece in order to saw off a part of the workpiece with the desired thickness. The machine controller of a sawing machine of the present type accordingly actuates the clamping jaws such that they clamp the workpiece in the zero position in which the front end side of the workpiece is located in the cutting plane, and move the clamped workpiece by way of a feeding movement from the zero position into a cutting position in which that point of the workpiece at which the workpiece is intended to be sawn off is located in the cutting plane.
The difficulty is thus that of moving the workpiece as precisely and yet quickly as possible into the zero position in the sawing machine when said workpiece is supplied to the sawing bench, in order to be able to begin the sawing operation.
A known possibility for solving this problem utilizes positioning a stop in the cutting plane, the workpiece butting against said stop when it is supplied such that as a result its end side comes to lie in the cutting plane. A disadvantage here is that the stop has to be removed for the sawing operation since otherwise a feeding movement into the cutting position would not be possible. At the same time, the stop has to be formed in a very resistant manner in order to be able to withstand the impulse of the butting workpiece. Finally, the workpiece has to be slowed down before the stop in order not to rebound at the stop on account of an elastic impact.
Another known solution to this problem utilizes detecting the end side of the workpiece by means of sensors on its way to the cutting plane and reporting this to the machine controller such that the latter can brake the workpiece in a timely manner before it reaches the cutting plane, and the workpiece finally remains in the zero position. In addition to the complexity which such a sensor system causes, it has been found that corresponding sensors are very susceptible to faults in the harsh environment of a sawing machine of the present type and have to be replaced frequently. This solution is therefore not optimal on account of the additional costs and on account of poor reliability.
A third known solution to the stated problem requires providing merely one sensor, specifically a light barrier, in the cutting plane and to convey the workpiece by eye into the sawing machine and onto the sawing bench, wherein the end side of the workpiece is conveyed beyond the cutting plane and thus the zero position. Then, the workpiece is clamped in the clamping jaws and the clamping jaws moved back until the light barrier arranged in the cutting plane has been cleared by the workpiece. At this moment, the movement is stopped, and so the zero position of the workpiece is reached, since at the moment at which the light barrier is cleared, the end side of the workpiece is located in the cutting plane.
A light barrier arranged in the cutting plane is very much less susceptible to faults than a complicated sensor system for controlling the conveying movement when conveying the workpiece into the sawing machine, but the constituent parts of the light barrier, which necessarily have to be arranged within the sawing machine, are also exposed to the harsh environmental conditions during operation of the sawing machine and accordingly likewise cannot always be operated in a reliably fault-free manner.